1. Field of the Invention
The present invention relates to a video camera comprising an inner focus lens.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing an inner focus lens 101 and the neighboring portion thereof. The inner focus lens 101 comprises a movable zoom lens 102, a focus lens 103, and fixed lenses 104. At one end of the moving range of the focus lens 103, a lens sensor 105 is disposed. The position at which the lens sensor 105 is disposed is referred to as an initial position. The lens sensor 105 is, for example, a photo interrupter. In addition, a step motor 106 is provided for moving the focus lens 103.
In the inner focus lens 101, the step motor 106 is used to move the focus lens 103. The initial position of the focus lens 103 is detected by the lens sensor 105. The absolute position of the focus lens 103 is obtained on the basis of the initial position so as to control the position of the focus lens 103.
In this video camera using such an inner focus lens 101, when the power thereof is turned on, as an initial process, the focus lens 103 is temporarily moved from the position at which the focus lens 103 is placed when the power is turned on (this position is named a first position) to one end of the inner focus lens 101 at which the lens sensor 105 is disposed. The lens sensor 105 detects whether or not the focus lens 103 arrived at the initial position. When the lens sensor 105 detects that the focus lens 103 arrived at the initial position, the initial process is completed. Thereafter, the absolute position of the focus lens 103 is obtained on the basis of the initial position so as to control the position of the focus lens 103.
That is, the operation of the conventional video camera is shown in FIG. 2. When the power of the video camera is turned on (at the step 110), a drive signal is input to the step motor 106 and thereby the focus lens 103 is moved from the first position to the initial position (at the step 111). The lens sensor 105 detects whether or not the focus lens 103 arrived at the initial position (at the step 112). When the lens sensor 105 detected that the the focus lens 103 arrived at the initial position, a controller (not shown in the figure) detects which mode, i.e. an focus mode or a manual focus mode, has been selected (at the step 113). When the auto focus mode has been selected, the focus lens 103 is moved to an in-focus position under auto focus control (at the step 114). When the manual focus mode has been selected, the user controls the position of the focus lens 103 (at the step 115). After a picture is photographed, the power of the video camera is turned off (at the step 116).
In the video camera using the conventional inner focus lens, when the power is turned off and then turned on, after the focus lens is moved back to the initial position for the initial process and then the focus lens is moved to the in-focus position or optimum position under auto focus control or manual focus control. Thus, when the same subject is photographed, if the power is turned off, the subject becomes out of focus at the initial position and thereby it takes a long time for adjusting the focus position.
In addition, in the video camera using the conventional inner focus lens, the lens sensor 105 is disposed at one end of the moving range of the focus lens 103. Thus, for example, if the in-focus position of the subject is at the other end of the moving range of the focus lens 103, when the power is turned on, the focus lens 103 is moved to the initial position at the end of the moving range thereof. Thereafter, the focus lens 103 is gradually moved from this initial position to the in-focus position at the other end of the moving range thereof under the auto focus control or manual focus control. Thus, it takes a long time for focusing the subject.